Carburizing is an effective method of increasing the surface hardness of low carbon, unalloyed, or low carbon, low alloy steels by increasing the carbon content in the exposed surface of steel. A carburized steel article, such as a gear, can transmit higher torques and have longer lives when they are carburized to produce a hard, wear resistant case. Typically, steel alloys are placed in an atmosphere containing carbon in an amount greater than the base carbon content of the steel and heated to a temperature above the austenite transformation temperature of steel. After the desired amount of carbon has been diffused into the article to a predetermined depth, hardness is induced by quenching.
Gas carburizing is a widely used method for carburizing steel. Being a diffusion process, carburizing is affected by the amount of alloying elements in the steel composition and the carburizing process parameters such as the carbon potential of the carburizing gas, the carburizing temperature, and the carburizing time.
Typical carburizing seeks to create a hardened case of martensite with some amount of retained austenite. It is normally considered unfavorable to form carbides during carburizing because they can weaken the material. Carbides can act as flaws that concentrate and localize strain and lead to subsurface cracks. In other applications, such as rolling and sliding applications, carbides are deliberately created to help refine grain size, reduce friction or improve pitting and scoring performance. In the few cases where carbides are intentionally created, a great deal of care is taken to control the carbide morphology and avoid high aspect ratio grain-boundary carbides that can drastically reduce performance. The depth of the carbide layer is typically a small fraction of the total carburized depth.
Another method of improving the performance and life of an article such as a gear tooth is to reduce operating contact stresses by improving geometric accuracy. Hard finishing of an article results in improved geometric accuracy and tighter manufacturing tolerances. Hard finishing, whether by grinding, honing, skiving, or some other technique, allows for the removal of distortion caused by heat treatment or some other manufacturing operation.
However, increasing demands for longer lives and higher power have exceeded the capabilities of either carbide carburized cases or hard finished faces. Hard finishing and carbide carburization have previously been two mutually exclusive techniques to improve rolling contact fatigue life. In the past, hard finishing would remove most, if not all, of the thin layers of carbides in the carburized case that may provide improved performance. The present invention seeks to combine these two aforementioned life improvement techniques to provide higher life and greater performance characteristics.
Some in the field have undertaken the task of trying to create carbides below the surface. Unfortunately, the focus has been on controlling the carbide morphology and creating fine spherical or spheroidal carbides through very specific processes while preventing the formation of massive non-spheroidal carbides. This technique, however, seeks to create fine spherical or spheroidal carbides in order to reduce the formation or break up the formation of the net shape or massive carbides in the austenite grain boundaries. Net shape or massive carbides in the austenite grain boundaries normally act as weak points or preferential crack points in the material.